The present invention relates to a servo motor controlling method for controlling the rotational speed and the current of servo motor under a digital control. More particularly, the present invention is directed to a servo motor controlling method in which the gain of a current control loop is changed dependent upon the rotational speed of the servo motor.
Due to improved performance of microprocessors and the reduced cost thereof, a digital controlling method for a servo motor has been extensively employed as a servo motor controlling method. In FIG. 4, there is shown a block diagram illustrating a servo motor controlling method performed under digital control. An operation unit 1 outputs a difference between a speed instruction VCMD and a speed feedback signal fb. An integrator 2 has a speed loop integration gain of k.sub.1v. An operation unit 3 outputs a current instruction value which is divided into three for three phases of a servo motor. Each of the divided values is fed to R, S and T phases. Since the arrangement of each of the phases is identical, only the R phase is shown in the figure and the remaining phases are omitted therein. Designated by numeral 4 is a speed loop proportional gain.
Denoted by reference numeral 10 is an R phase current controlling loop. An operation unit 11 outputs a difference between an R phase feedback current Ir and the current instruction. An integrator 12 has a current loop integration gain of k.sub.1. An operation unit 13 outputs a difference between the output of the integrator 12 and a current loop proportional gain k.sub.2 designated by reference numeral 14. An operation unit 15 outputs a difference between a counter electromotive voltage E(S) of the servo motor and the output of the operation unit 13 13. A first order delay element 16 includes a winding resistance R of the servo motor and an inductance L thereof. The output of the first-order delay element 16 is a current in the servo motor.
A torque constant K.sub.t is designated by reference numeral 17 and its output is a torque. A motor 18 imposes an inertia Jm which is an integrator in terms of a numerical equation. The output of the motor 18 defines the rotational speed of the servo motor and outputs a speed feedback signal fb.
In such a servo motor controlling method performed under digital control, however, a period of time required for the software processing of the digital control is not negligible when the servo motor has reached a high speed rotational state. The delay of the processing time causes a lower current loop gain. Further, the electromotive force of the servo motor becomes high and the current loop gain is brought to a decreased status. For these reasons. the high speed rotations cannot be attained and a storage of torque results In contrast, the constant rising of the current loop gain causes oscillation of the current loop to occur at the time when the motor is rotating at a low speed or is stopped.